Negative bias voltage problem - 50w 1986 bass spec

[leonard.gulyas]

Dear Members,

I would like to ask your help in my new build.

The specs are the following:

50W
1986 bass spec
Marstran 1202-118 PT
Marstran 784-139 OT
Marstran C1999 Choke
Larry star grounding
everything else is according to the kit instructions

The problem is I can't get the required -20 ~ -50 VDC interval for the negative bias supply. I can only get -14VDC when the bias pot is on max and -12VDC when is on minimal. The wall voltage is around 236V so the voltage selector is set to 240V.

Here are the initial voltage reading for Testing Part1: Powering up:



Here are some more pictures for troubleshooting.





Thanks in advance.

Regards

[neikeel]

Replace the 150k feed resistor (as used in JTM45RIs) with the more correct 220k for a JMP50 as that is what Brian's PT is

Otherwise looks pretty neat, but you might find that particular bright cap a bit spiky?

[V8]

Nice looking amp, what is that yellow wire connected to the left bias cap/15k/47k junction ?

David

[leonard.gulyas]

Replace the 150k feed resistor (as used in JTM45RIs) with the more correct 220k for a JMP50 as that is what Brian's PT is

Otherwise looks pretty neat, but you might find that particular bright cap a bit spiky?

Dear neikeel,

I changed the 150k (bias supply dropping resistor) with a 220k one but nothings changed. Am I missing something?

Regards

[leonard.gulyas]

Nice looking amp, what is that yellow wire connected to the left bias cap/15k/47k junction ?

David

Dear David,

You are right the yellow wire (screen grid) along with the choke wire should be connected to the 4th terminal from the right. So I corrected it and right now the bias voltage reads -29 ~ -38 VDC. I hope this is right.

Neikeel, what bright cap do you recommend instead of the 250K?

Regards

[leonard.gulyas]

So I changed back the resistor to 150K and the negative bias voltage range is -36 ~ -51 VDC.

Here is the voltage chart with the EL34s biased to around 70%.



Thanks for all the help

Regards

[neikeel]

Well spotted V8!

- did you relocate the thin black choke wire to the B+ line with the yellow wire as well?

- 220k is correct for that PT but

- a 100pF cap is sweetest with a 1meg volume pot, 250pF to 1000pF is spiky (to my ears) and 5000pF is just a sonic assault after the volume passes 2.5 on the dial (I prefer that in a lead amp).

[leonard.gulyas]

Well spotted V8!

- did you relocate the thin black choke wire to the B+ line with the yellow wire as well?

- 220k is correct for that PT but

- a 100pF cap is sweetest with a 1meg volume pot, 250pF to 1000pF is spiky (to my ears) and 5000pF is just a sonic assault after the volume passes 2.5 on the dial (I prefer that in a lead amp).

Yeah I did the relocation of the cables. I will definitely try a 100pF cap and let you know of the results but right now I have to find a decent cab for my amp.

Regards

[Sxandy]

I also have mustards in my two builds for tonal reasons but in the cathode position you may not notice much difference between brands. I am running several different brands of caps in that position and cannot tell much difference in tone between them. Changing the value will make a noticeable change though, so you may want to only do one cathode at a time and try the amp first before changing both. Let us know how you like those values in that spot.